As a result of experience with long term operation and multiple reloads of nuclear fuel elements, it has been found that certain operating conditions arise which tend to reduce energy output per unit of fuel obtainable and thereby affect operating costs and efficiencies in an undesirable manner.
One of the problems in connection with the operation of nuclear reactors is the accumulation of debris of various sizes, which may occur during original construction, subsequent operation or during repair. Examples of such debris include small fasteners, metal clips, welding slag, and small pieces of wire. During the operation of nuclear reactors, the debris which may be present in the nuclear reactor can be carried by the cooling water and can impact upon fuel assembly components. The repeated interaction of such debris and fuel assembly components can result in fretting damage to the components. Certain sizes of this type of debris are particularly troublesome, since that debris is likely to be carried by cooling water to the area near the bottom (lower ends) of the fuel rods. Some of the debris can be caught between the fuel rods and other fuel assembly components. The debris vibrates in the moving coolant and impacts principally upon the lower end of the fuel rods, ultimately abrading and causing fretting wear of the fuel rod cladding at that point. This type of wear is recognized as a significant cause of fuel failures which may release fuel and fission products into the coolant leading to the premature withdrawal from service of the fuel assembly or costly fuel rod replacement.
Attempts have been made in the past to mitigate the affect of such debris on the fuel rods by incorporating integral filter mechanisms during the manufacture of new fuel assemblies containing unirradiated nuclear fuel. However, the problem of protecting irradiated nuclear fuel rods in a nuclear fuel assembly has been more problematic and costly. While the incorporation of debris resistant features in new fuel assemblies during their manufacture is known, it would be an advantage over the prior art to provide debris resistance for irradiated fuel assemblies not so equipped or requiring further debris resistant features. The physical configuration of typical BWR assemblies, however, precludes retrofitting of debris resistant devices to the irradiated fuel assembly without incurring unacceptably high costs. This is due particularly (a) to the configuration of the lower tie plate where debris resistant features must be incorporated to the fuel assembly, and (b) to the need to modify the fuel assembly remotely, typically under several feet of water.